Electrode system particularly semiconductor electrode system and method of producing the same

ABSTRACT

An electrode system comprising a semiconductive granular layer substantially one grain in thickness is manufactured by embedding a layer of this semiconductive material in an insulating material, e.g., polyurethane so that the peaks of the grains extend substantially above the insulating material i.e., the spaces between the grains are filled to a level well below the peaks of the grains. An electrode is formed on this layer by depositing an electrically conducting layer. The so-coated layer is thereafter subjected to an abrasive on a carrier comprising grains whose average diameter is smaller than the average diameter of the electrically active grains and whose diameter exceeds the average spacing between the active grains to remove only the electrode layer covering the exposed grain peaks without abrading the grains themselves. The grain size of the abrasive is critical to avoid grains of the abrasive being lodged in the depressions between the grains of the semiconductive material and to avoid removing portions of the semiconductive grains.

United States Patent [54] ELECTRODE SYSTEM PARTICULARLY SEMICONDUCTOR ELECTRODE SYSTEM AND METHOD OF PRODUCING THE SAME 8Claims,8l)rawing Figs.

[52] U.S.Cl 117/210, 117/8,117/25,l17/26,117/212,1l'7/215 [51] lnt.Cl B44d1/18, 1-10lj H00 [50] FieldotSearch ..117/210,8,

[56] References Cited UNITED STATES PATENTS 2,201,196 5/1940 Williamson 1 17/26 Primary Examiner--Alfred L. Leavitt Assistant Examiner-M. F. Esposito Attorney-Frank R. Trifari ABSTRACT: An electrode system comprising a semiconductive granular layer substantially one grain in thickness is manufactured by embedding a layer of this semiconductive material in an insulating material, e.g., polyurethane so that the peaks of the grains extend substantially above the insulating material i.e the spaces between the grains are filled to a level well below the peaks of the grains. An electrode is formed on this layer by depositing an electrically conducting layer. The so-coated layer is thereafter subjected to an abra sive on a carrier comprising grains whose average diameter is smaller than the average diameter of the electrically active grains and whose diameter exceeds the average spacing between the active grains to remove only the electrode layer covering the exposed grain peaks without abrading the grains themselves. The grain size of the abrasive is critical to avoid grains of the abrasive being lodged in the depressions between the grains of the semiconductive material and to avoid removing portions of the semiconductive grains.

PATENTEDunv 1s l97| SHEET 1 BF 2 III MI. I

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FIGA

INVENTOR. TIES 5 .TE VELDE AGENT ELECTRODE SYSTEM PARTICULARLY SEMICONDUCTOR ELECTRODE SYSTEM AND METHOD OF PRODUCING THE SAME The invention relates to a method of manufacturing an electrode system comprising a granular layer, for example, a semiconductor granular layer substantially of the thickness of one grain, the grains being embedded in an insulating filling substance, said granular layer being coated by at least one electrode layer, which is in contact with the grains and which consists of a pattern of coherent, electrically good-conducting domains between the grains on the filling substance and of electrically usually more poorly conductive domains on the grains in which method, in order to obtain said electrode layer, a coherent, uninterrupted electrically good-conducting electrode layer is applied to the granular layer, after which portions of the electrode layer on the grains are selectively removed. The invention furthermore relates to an electrode system manufactured by the method according to the invention.

Such an electrode system, as well as a method of manufacturing the same, is the subject matter of a copending US. Pat. application, Ser. No. 569,204, filed Aug. 1, 1966, now U.S. Pat. No. 3,480,818. Herein is described a method of the kind set forth, in which a good-conducting electrode layer is applied to the whole granular layer, after which either by a selective etching process or by using photoresist methods this goodconducting electrode layer is selectively removed from the heads of the grains. If desired, a second electrode layer may be applied to the grains.

Electrode systems of the kind set forth may comprise radiation-sensitive grains and may be used as radiation detectors, in which case radiation energy is incident to the photosensitive granular layer, in which it produces electric voltage or impedance differences, which can be measured by means of electrodes provided on the granular layer, at least one of which electrodes has to be transparent to the incident radiation. Examples of these uses are inter alia photoresistors and photocells for exposure meters; such electrode systems may furthermore be employed for converting radiation energy into electric energy, inter alia in so-called solar batteries. For converting electric energy into radiation energy, in which case in the grains radiation can be produced for example by recombination of charge carriers at a PN junction or by other forms of electroluminescence, electrode systems comprising a granular layer are interesting.

In all these cases not only grains having approximately the same dimensions in all directions but also grains in the form of scales or needles may be used. The electrode systems of the kind set forth involve often the problem of having to provide on the granular layer at least one electrode layer for current input or current output, which electrode layer has to satisfy conflicting requirements. On the one hand the electrode layer should have a low sheet resistance, whereas on the other hand at the contact area with the grains the electrode should have properties which may contradict said low sheet resistance.

If the electrode system comprising a granular layer is, for example, an electro-optical device, at lease one of the electrodes applied to the granular layer has to be transparent, at least on the grains, to the radiation incident to the grains or emanating from the grains. in connection with the usually poor conductivity of such transparent electrode layers an electrode layer of unhomogeneous structure is desired, in which l coherent, good-conducting domains between the grains and less good-conducting domains of the desired radiation-transparent properties on top of the grains are provided. In view of the required transparency it may in some cases be desirable to omit partly the electrode layer on the grains.

The invention has for its object to provide a particularly rapid method for applying such an unhomogeneous electrode layer.

The invention is based on the discovery that in the case of a granular layer, on the electrode side of which the filling substance exhibits subsidences between the grains, parts of the electrode layer located on the grains can be selectively removed by abrading this side of the electrode-covered granular layer, while the parts of the electrode layer located in the subsidences of the filling substance between the grains are substantially not affected by the abrasive.

According to the invention a method of manufacturing an electrode system of the kind referred to above is characterized in that the manufacture starts from a granular layer, the filling substance between the grains having a thickness considerably smaller than the average grain thickness, so that at lease on one side the-granular layer exhibits subsidences or hollows or depressions between the grains and in that on said side the granular layer is coated by an electrically good-conducting electrode layer, after which by abrading the granular layer on said side only portions of the electrode layer on the grains are removed owing to the subsidences between the grains.

Abrasion may be carried out by various grinding or abrading agents. It is advantageous to use an abrasive whose grains have such a size that they cannot reach the bottom of the subsidence. Therefore a preferred embodiment of the method according to the invention is characterized in that the granular layer is abraded by means of an abrasive whose grains have a diameter exceeding the average distance between the grains on the granular layer, so that they cannot reach down to the bottom of the depressions. It is preferred to use an abrasive, the grain diameter of which is more than twice and less than five times the average distance between the grains of the granular layer. It is furthermore advantageous to use an abrasive whose grains have a considerably smaller diameter than the average grain diameter of the granular layer and a further, important embodiment of the method according to the invention is characterized in that the granular layer is abraded with the aid of an abrasive whose grains have a diameter which is considerably smaller than the average grain diameter of the granular layer, said abrasive being applied to a carrier. In this case the carrier is desired for preventing the abrading grains from attacking the electrode layer in the subsidences between the grains of the granular layer. The carrier should preferably have such a flexibility that it can follow the shape of the granular layer, so that in practice each grain of the granular layer comes into contact with the abrasive.

lt is advantageous to use an abrasive in this case, the grain diameter of which is less than one-fifth, preferably less than one-tenth of the average grain diameter of the granular layer.

ln the above embodiments of the method according to the invention it is possible to start with a granular layer, the filling substance of which extends over the grains. The parts of the filling substance located on the heads of the grains is then removed simultaneously with the electrode layer by abrasion.

However, owing to the disadvantages involved herein, for ex-.

ample, the plastic properties of most filling substances, which are therefore less suitable for an abrading or grinding process, it is preferred to start with a granular layer whose grains protrude from the filling substance on the side of the electrode layer, so that the abrasive need not come into contact with the filling substance.

It is advantageous to start in this case with a granular layer obtained by embedding the grains in a hardening filling substance which contracts between the grains so that parts of the grains are made free of the filling substance.

In an important, preferred embodiment of the method according to the invention a granular layer is used, the grains of which consist of cadmium sulfide and the filling substance of which consists of polyurethane.

In a further important, preferred embodiment of the method according to the invention the abrasion produces openings in the electrode layers on the heads of the grains, while the contact between the electrode layer and the grains is maintained.

This method has the advantage that in a single operation an electrode is formed, which is good conducting between the grains, while on the heads of the grains the incident and the emerging radiations are not hindered, while the resultant contact, though only present on part of the grain surface, is in many cases sufficient. For improving the contact it is advantageous to subject the granular layer, subsequent to abrasion, to an ion or electron bombardment.

Although a satisfactory contact can be obtained in this manner without subjecting the grain parts free of the electrode layer to a further process, it will be desirable in other cases to cover the free grain parts with further contact material in order to obtain given, desired contact properties. The initial electrode layer may, if desired, be completely removed from the grains. A further preferred embodiment of the method according to the invention is characterized in that subsequent to the removal of only the parts of the electrode layer from the heads of the grains a second electrode layer, conductively connected to the first electrode layer, is applied to at least the free grain portions. In those cases in which the electrode layer on the grains has to be transparent to incident or emerging radiation a second electrode layer is used, in an important, preferred embodiment, which layer has a greater transparency than the first electrode layer for electromagnetic radiation to be emitted by the grains or for a radiation to which the grains are sensitive.

The method described above may be used with different combinations of grains and electrode materials. The invention is, however, particularly important for the manufacture of electrode systems having a granular layer whose grains consist mainly of photoconductive sulfides and/or selenides of cadmium and zinc, while the grains are covered by an electrode layer containing indium or an indium alloy. Between the electrode layer and the grains an ohmic contact is then established.

The electrode layer may be applied without preliminary treatment of the granular layer. In order to obtain a satisfactory ohmic contact it is, however, often desired to subject the granular layer to an ion or electron bombardment prior to the application of the electrode layer.

The invention furthermore relates to an electrode system manufactured by the method according to the invention.

The invention will now be described more fully with reference to a few embodiments shown in the drawing, in which:

FIGS. 1 to 3 illustrate diagrammatically in cross-sectional views consecutive steps of the manufacture of part of a photoresistor according to a method according to the invention,

FIG. 4 is a diagrammatic plan view of the photoresistor the manufacture of which is illustrated in a cross-sectional view taken on the line [-1 in FIGS. I to 3 and FIGS. to 8 illustrate diagrammatically in cross-sectional views consecutive stages of the manufacture of part of a solar cell obtained by a method according to the invention.

A first embodiment of a method of manufacturing an electrode system will now be described with reference to FIGS. 1 to 4; said system comprises a granular layer 1, 2, having grains I (FIG. 3) of a semiconductor material having substantially the thickness of one grain, the grains being embedded in an insulating filling substance 2; this granular layer 1, 2 is covered by at least one electrode layer 3, 4 which is in contact with the grains 1 and which consists of a pattern of coherent, electrically good-conducting domains 3 between the grains on the filling substance 2 and electrically less good-conducting domains 4 on the heads of the grains; in order to obtain said electrode layer on the granular layer 1, 2 a coherent, uninterrupted, electrically good-conducting electrode layer 3 (see FIG. 1) is applied, after which the parts 5 of the electrode layer 3 located on the grains are selectively removed. This general structure has been described in detail in my copending application, Ser. No. 569,204, filed Aug. 1, 1966, now U.S. Pat No. 3,480,818 whose contents are hereby incorporated by reference. An illustrative method for making same is as follows.

The starting material is again a granular layer 1, 2 stuck by an adhesive layer 7 (FIG. 1) to support 8 and cohering by means of a filling substance 2, operating as a binder, and having granular portion 9 free of the filling substance; this granular layer is covered by an electrode layer 3. The grains 1 consist of photoconductive cadmium sulfide, activated by about 10 to 10" percent by weight of copper and gallium.

Such a granular layer may be obtained by various means. In connection with the necessity of forming subsidences between the grains, the following, very appropriate method is carried out.

A carrier, for example, a glass plate 1 is coated, by immersion in a solution of gelatine in water, with a gelatine layer 7 of a few microns thick. Photoconductive cadmium sulfide grains of a diameter of about 35 u are sunk into the still-swollen gelatine layer, after which the gelatine is hardened by drying and the grains not adhering to the support are removed. Then polyurethane is applied as a binder between the grains on the gelatine. This may be carried out by dipping the carrier with the granular layer in a solution of materials known commercially under the trademarks of Desmophen" and Desmodur" in ethylacetate; when pulling the plate up, a thin layer is left on the granular layer, which layer is converted by a hardening process of, for example, 8 hours, at 75, into polyurethane. Then the initially less viscous mixture withdraws from the heads of the grains, so that subsequent to hardening a granular layer is obtained, the grains of which project from the filling substance on the side remote from the carrier, while the filling substance between the grains has a thickness which is considerably smaller than the average grain thickness, so that on the side remote from the carrier the granular layer exhibits subsidences between the grains, which has also been described in my copending application, Ser. No. 629,999, filed Apr. 1 l, 1967.

The resultant granular layer may be directly coated by vapor deposition by an electrode layer 3. In order to improve the contact between the grains and the electrode layer, the granular layer is preferably subjected to an ion or electron bombardment prior to the application of the electrode layer 3. In the present embodiment this is achieved by exposing the granular layer to a gas discharge for about 4 minutes at a voltage of l kv. (discharge current about 50 ma. with an electrode surface of I00 cmF). Other suitable techniques are described in my copending application, Ser. No. 630,026, filed Apr. I 1, I967, now U.S. Pat. No. 3,481,030. The side of the granular layer remote from the carrier is then covered by the electrode layer 3, which establishes a substantially ohmic contact with the cadmium sulfide grains; for example, an indium layer of a thickness of 5,000 A. is applied from the vapor phase.

On the side remote from the carrier (see FIG. 1) the granular layer is abraded by means of grains I0 of alumina having a diameter lying between about I50 p. and 250 t. This diameter is greater than twice and smaller than five times the average distance between the grains of the layer, in this case 70 to I00 ;1.. The abrasive grains 10 are used in the dry state and rubbed by means of a piece of cloth or another soft object 16 (see FIG. 1) with a light pressure across the granular layer. Owing to their size the abrasive grains cannot reach the coherent portions 3 of the electrode layer located in the subsidences between the grains on the filling substance, so that only the parts of the electrode layer on the heads of the grains 5 are removed. Thus holes 4 are made on the grains (see FIGS. 2 and 4) in the electrode layer; while portions 6 of the electrode layer remain in contact with the grains at the edges of the holes. After the formation of the holes, which is checked by a microscope, abrading is ceased and, if desired, any residues of polyurethane on the abraded grain parts may be removed by a dissolving or etching process. In order to improve the contact on the grains the abraded side of the granular layer is then again exposed to a gas discharge.

Then (see FIG. 3) an approximately 50 LL thick, hardening radiation-pervious, flexible layer ll of a synthetic resin is then applied to the granular layer, for example, of polyurethane, and after hardening of said layer the granular layer is removed from the carrier 8 by dissolving the gelatine layer 7 in water, so that on the side of the carrier the grain portions 12 are free. After the gelatine residues are removed inwater, the free side of the granular layer is exposed to a gas discharge and a second electrode layer 13 (see FIG. 3) is applied thereto by vapor deposition of indium to a thickness of about 5,000 A.

In this manner a photoresistor formed by a flexible sheet is formed as is shown in FIG. 3 in a cross-sectional view and in FIG. 4 in a plan view.

On the electrode layer 13 and on a portion 14 of the electrode layer 3, 4, 6 free of the layer 11 contacts may be provided, between which the impedance of the granular layer can be measured. Through the synthetic resin layer 11 and the holes 4 a radiation 15 can strike the grain portions not covered by the electrode layer 3, 4, 6 while all grains are connected in parallel between the electrode layer 13 and the portions 6 of the electrode layer 3, 4, 6 which establish a substantially ohmic contact with the grains 1.

A second example of the method according to the invention will be described with reference to FIGS. 5 to 8 for the manufacture of a solar cell. The same reference numerals of the two embodiments designate corresponding parts. The starting material is again (see FIG. 5) a granular layer 1, 2 stuck by means of a gelatine layer 7 to a carrier 8 and consisting of grains 1 of photoconductive cadmium sulfide of an average diameter of about 35 u, embedded in a binder 2 of polyurethane and manufactured in the manner described above; on the side remote from the carrier the granular layer exhibits subsidences between the grains and is coated completely, for example by vapor deposition, with an electrode layer 3, 5. The electrode layer 3, 5 consists in this example of about 0.1 1. thick copper layer.

The granular layer is then abraded on the side remote from the carrier (see FIG. 1) by means of abrasive grains 21 of alumina of a diameter of about 5 p, applied to a flexible support 22 of a sheet of silicon rubber of about 0.2 mm. thickness. The diameter of the abrasive grains is smaller than one-fifth of the mean grain diameter of the granular layer. Under given conditions it may be advantageous to use still smaller abrasive grains of a diameter smaller than one-tenth of the average diameter of the grains of the granular layer. The abrasive grains 21 may be applied to the support 22, for example, by pouring out the silicon rubber in the liquid state on a glass plate and strewing alumina grains after which the silicon rubber is caused to harden to fonn a flexible sheet, in which the abrasive grains are partially embedded. Abrasion is performed by arranging a cushion 23 of foam plastics or another very elastic material on the carrier 22 with the abrasive grains (FIG. 5) and thereon a nonelastic flat object 24 and by rubbing the carrier of the abrasive grains by hand of by a machine with slight pressure across the granular layer. The carrier 22 follows the contours of the granular layer and only the portions of the electrode layer 3, 5 on the heads of the grains are removed so that (see (FIG. 6) a structure similar to that of FIG. 2 of the first embodiment is obtained. Portions 6 of the electrode layer remain in contact with the grains 1, although this is not required in this example with a view to the electrode layer 25 to be applied afterwards.

On the side remote from the carrier the resultant layer is coated with a second, radiationpervious, less good-conductive electrode layer 25 of copper by vapor deposition to a thickness of about 100 A., which layer establishes a contact with the portions 3 of the initial copper layer 3, 5 and a rectifying contact with the grains 1. In this example, in which a rectifying contact instead of an ohmic contact is formed with grains, a gas discharge prior to the application of the electrode layer 25 is, of course, omitted. Subsequently, the layer 25 is coated with a radiation-pervious, hardening, preferably flexible plastics layer 11, for example, of polyurethane of a thickness of about 50 ;1.. After hardening of the layer 11 the granular layer is removed from the carrier by dissolving the gelatine layer 7 in water, so that on the side of the carrier grain portions are exposed. These exposed sides of the grains are subjected to a gas discharge and (see FIG. 8) an electrode layer 13 is applied by vapor deposition of an about 5,000 A. thick indium layer. This electrode layer 13 establishes an ohmic contact with the grains.

In this manner (see FIG. 3) a solar cell is obtained, which also has the shape of a flexible sheet; radiation 26 can strike through the pervious plastics layer 11 and the pervious copper layer 25, the rectifying copper cadmium sulfide contact. The electrode layer 13 and a portion 14 of the electrode layer 3, 25 clear of the layer 11 may be provided with contacts, from which the photovoltage produced by the incident radiation can be derived.

It will be obvious that the invention is not restricted to the above examples and many variants are possible for those skilled in the art. Under given conditions it will be possible to start from a granular layer whose grains do not project from the filling substance, in which case by abrasion not only the portions of the electrode layer located on the grains but also the subjacent filling substance are removed. Since in this case the remaining portions of the electrode layer are no longer in contact with the grains, it is necessary to apply a second electrode layer to the relevant side of the granular layer as described with reference to the second embodiment. Instead of photoresistors and photocells electrode systems may be manufactured which comprise granular layers capable of emitting injection recombination radiation under the action of an applied voltage. Moreover, grains of other material than cadmium sulfide may be employed and in dependence upon the use of the resultant electrode system they need not be photoconductive, for example, for the manufacture of diodes, capacitors, bolometers, nonlinear resistors and the like. As stated above, filling substances of quite different materials may be used, for example epoxyresins or photohardening lacquers in accordance with the use and the technique used for the formation of the starting granular layer, while also the covering layer 11 (see FIGS. 3, 7 and 8) may consist not only of polyurethane but also of, for example, methylmethacrylate or another hardening synthetic resin pervious or not pervious to radiation.

Under given conditions it may be advantageous to omit the electrode layer 13 (see FIGS. 3 and 8) and replace same by a flow of charged particles, for example, ions or electrons, which are incident to the granular layer and convey the charge. When, for example, the granular layer is used as a photoconductive layer in xerography, an electrode layer may be replaced by a gas discharge for local discharge of the photoconductve layer.

Finally it may be desirable for given uses not to remove the granular layer from the carrier 8; in this case an electrode layer may be provided previously between the carrier and the granular layer.

What is claimed is:

1. In a method of manufacturing an electrical device comprising a granular layer of electrically active grains having electrode means therefor with at least one of the electrodes having highly electrically conductive portions between the grains, the improvement comprising the steps of embedding a layer of said grains substantially the thickness of one grain in an electrically insulating filling substance such that at least at one side of the layer the filling substance level between the grains lies below the grain peaks, applying on the said one side a coating of an electrically conductive material to form a continuous electrode layer of high electrical conductivity having higher portions on the grain peaks and lower portions on the intervening filler substance, and thereafter subjecting the electrode coated side of the layer to an abrasive comprising on a carrier grains whose diameter is smaller than the average diameter of the electrically active grains in the granular layer and whose diameter exceeds the average spacing between the electrically active grains in the granular layer to remove only the higher electrode layer portions coating the grain peaks thereby exposing the grain surfaces without abrading the grains and leaving in position lower electrode layer portions on the filler substance.

2. A method as set forth in claim 1 wherein the abrasive grains have a diameter between two and five times the average spacing between electrically active grains.

6. A method as set forth in claim 1 wherein electrode means are provided on the opposite side of the granular layer.

7. A method as set forth in claim 1 wherein the grains comprise cadmium sulfide, zinc sulfide, cadmium selenide, or zinc selenide.

8. A method as set forth in claim 7 wherein the metal electrode comprises indium and the filling substance is polyurethane.

Patent No.

Dated November lg 1971 Inventofls) TIES SIEBOLT TE VELDE It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

"lease" to read -least--.

"10 to read --1O and "10 to read --10' to read 8-.

Signed and sealed this 12th day of March 197J.

Column 2, line 9, change Column 4, line 3, change change Column 6, line 1, change (SEAL) Attest:

EDWARD M. FLETCHER,JR. Attesting Officer C. MARSHALL DANN Commissioner of Patents 

2. A method as set forth in claim 1 wherein the abrasive grains have a diameter between two and five times the average spacing between electrically active grains.
 3. A method as set forth in claim 1 wherein the abrasive grains have a diameter between one-fifth and one-tenth of the average diameter of the electrically active grains.
 4. A method as claimed in claim 1 wherein, subsequent to the abrading step, the granular layer is suBjected to ion or electron bombardment.
 5. A method as claimed in claim 1 wherein, subsequent to the abrading step, a second electrically conductive coating is applied onto the exposed grain surfaces and electrode layer portions left in position.
 6. A method as set forth in claim 1 wherein electrode means are provided on the opposite side of the granular layer.
 7. A method as set forth in claim 1 wherein the grains comprise cadmium sulfide, zinc sulfide, cadmium selenide, or zinc selenide.
 8. A method as set forth in claim 7 wherein the metal electrode comprises indium and the filling substance is polyurethane. 